1. Field of the Invention
The present invention relates to a method for manufacturing a bipolar transistor on a substrate of semiconductor material in which a collector layer, a base layer and an emitter layer for the formation of the collector, the base and the emitter, respectively, are grown on top of one another.
2. Description of the Prior Art
In gallium arsenide-gallium aluminum arsenide (GaAs-GaAlAs) heterobipolar transistors, the transit frequency and the maximum oscillation frequency are greatly influenced by parasitic capacitances and series resistances. Examples of parasitic capacitances are the capacitances between the emitter layer and the base layer or between the base layer and the collector layer outside of the inner transistor region. In order to achieve high values for the maximum oscillation frequency, the path resistance of the base and the base-collector capacitance must be minimized.
It is fundamentally possible to reduce parasitic capacitances by miniaturizing structures, particularly by reducing the distance between the base and the emitter contacts or, respectively, between the base and the collector contacts. A further possibility of reducing the extrinsic base-collector capacitance is offered by an isolation implantation (for example, oxygen) in the extrinsic base-collector region and disclosed by P. M. Asbeck, D. L. Miller, R. J. Anderson and F. H. Eisen, in the IEEE Electron Device Letters publication, Vol. 5, 1984, pp. 310-312. The high implantation energies needed are thereby disadvantageous. These high implantation energies require masks of metal. Moreover, the implantations also cause damage in the upper base layers, this damage disadvantageously modifying the transistor properties.